Mapping gravitational-wave backgrounds using methods from CMB analysis: Application to pulsar timing arrays

The authors describe how the formalism used to describe CMB polarisation can be used to construct maps of the gravitational-wave background using pulsar timing arrays (PTAs). While the ''gradient'' part of the background can be constrained by timing sufficiently many pulsars (top panel), the ''curl'' part of the background (bottom panel) is invisible to PTAs.